1. Field of the Invention
The present invention generally relates to a varactor. More particularly, the present invention generally relates to a varactor and a differential varactor having large capacitance tuning range and large voltage tuning range.
2. Description of Related Art
In a conventional communication system such as a television program system, the data signal is tuned and being transported by the high frequency carrier wave. Since the signal of the carrier wave is dependent on the frequency, a variety of data signals with different frequency may be transported by different carrier waves. Therefore, the receiver of the communication system needs a voltage controlled oscillator (VCO) to separate the data signals from the carrier waves. A conventional voltage controlled oscillator (VCO) generally includes an inductance-capacitance (LC) circuit constructed by varactor and inductor. Thus, the oscillation frequency of the voltage controlled oscillator (VCO) may be tuned by tuning the voltage of the varactor to change the capacitance of the varactor.
The conventional varactor may be classified into MOS varactor and junction varactor. The MOS varactor is constructed by the metal oxide semiconductor (MOS) transistor structure. The junction varactor is constructed by interlaced p-type doped region and n-type doped region. The MOS varactor has a large capacitance tuning range defined as (maximum capacitance−minimum capacitance)/minimum capacitance), but the voltage tuning range of the tuning range is very small (in general, only about 1V). In other words, a fine voltage tuning may change the capacitance drastically. In summary, although the tuning range of the capacitance of MOS varactor is large, but the voltage tuning range of the capacitance is too small and therefore it is difficult to precisely control the voltgae capacitance of MOS varactor. On the other hand, the junction varactor has a large voltage tuning range (in general, larger than 2V), but the capacitance tuning range is not large enough, therefore the application of the junction varactor is limited. Accordingly, a varactor having a large voltage tuning range and large tuning capacitance is highly desirable.